Having retired from Monte Carlo and not been entered for Sweden, Sordo's zero-point championship position meant he was a very favourable 10th of the 11 World Rally Cars on the road for Mexico's first gravel stages.
Last year's Mexico winner Kris Meeke - seventh on the road - hit the front for Citroen initially on the Duarte-Derramadero stage, but back-to-back stage wins on the following El Chocolate and Ortega stages thrust Sordo to the front while Meeke felt his choice of two hard front tyres might have proved incorrect.
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Meeke dropped to third behind Elfyn Evans - who was eighth in the start order - on El Chocolate, but a high-speed trip off the road on Ortega cost Evans nearly three minutes and dropped him to 12th.
That elevated Meeke and Loeb to second and third, with Sordo leading by 16.6 seconds heading to service.
Loeb closed to within 1.3s of Meeke on the Leon street stage, helped by a minor Meeke error in which his Citroen went up on two wheels and drove over a large pot plant on the apex of a hairpin.
Ott Tanak is close behind the two Citroens in the only Toyota not struck by a repeat of last year's overheating problems. Team-mates Jari-Matti Latvala and Esapekka Lappi have struggled badly with power losses and are back in seventh and 10th.
Championship leader Thierry Neuville had to grapple with opening the dusty road, but was staying in touch with the front group in seventh until the Leon street stage just before service.
He slowed and reported a potential engine problem, and his Hyundai's driver's side door also repeatedly flapped open during the stage, dropping him to ninth behind Leon pacesetter Teemu Suninen.
Although Sebastien Ogier is in the unusual position of not opening the road, he felt running second was still a heavy disadvantage and he is fifth - just 1.3s ahead of Andreas Mikkelsen, who was not totally happy with his Hyundai's handling.
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